Difference between 2010 EV and 2015 EV?

I'm wondering if anybody knows why it's harder to activate EV mode on the 2010 vs 2015? My 2015, I can usually press the EV button almost any time and it'll go into EV. However my 2010, seems like I'm always locked out of the function. When I press it, it shows EV not ready about 80% of the time. The HV battery is usually charged up when I choose the selection. Very Strange!

I was thinking they did something in 2015 that lets the EV engage easier than 2010 models.